halides except phosphorus trifluoride. PX + 3H O H PO + 3HX Hydrogen bromide may be obtained by adding bromine dropwise to a paste of red phosphorous and water while hydrogen iodide is conveniently produced by adding water dropwise to a mixture of red phosphorous and iodine. 2P + 3X 2PX 2PX + 3H O H PO + 3HX (where X=Br or I) Any halogen vapours which escapes with the hydrogen halide is removed by passing the gases through a column of moist red phosphorous. From covalent hydrides: Halogens are reduced to hydrogen halides by hydrogen sulphide.
H S + X 2HX + S Hydrogen chloride is obtained as a by-product of the reactions between hydrocarbon of halogens. XII XII - - - - Table . : General Properties: HF HCl HBr HI Bond dissociation enthalphy(KJmol - ) + + + + % of ionic character In line with the decreasing bond dissociation enthalpy, the thermal stability of hydrogen halides decreases from fluoride to iodide. For example, Hydrogen iodide decomposes at ° C while hydrogen fluoride and hydrogen chloride are stable at this temperature.
At room temperature, hydrogen halides are gases but hydrogen fluoride can be readily liquefied. The gases are colourless but, with moist air gives white fumes due to the production of droplets of hydrohalic acid. In HF, due to the presence of strong hydrogen bond it has high melting and boiling points. This effect is absent in other hydrogen halides.
Acidic properties: The hydrogen halides are extremely soluble in water due to the ionisation. HX + H O H O + X (X – F, Cl, Br, or I) Solutions of hydrogen halides are therefore acidic and known as hydrohalic acids. Hydrochloric, hydrobromic and hydroiodic acids are almost completely ionised and are therefore strong acids but HF is a weak acid i.e. .1mM solution is only % ionised, but in 5M and 15M solution HF is stronger acid due to the equilibrium.
HF + H O H O + F HF + F HF ⇀ ↽ ⇀ ↽ At